1. Field of the Invention
The present invention relates to a process of producing a light-reflective film having four or more light-reflective layers of a fixed cholesteric liquid crystal phase, which may be attached to windowpanes or the like. The present invention relates also to the light-reflective films produced according to the process of the present invention.
2. Background Art
With the recent increase in interest in environment and energy-related issues, the needs for energy-saving industrial products are increasing; and as one of them, glass and film are desired that are effective for heat shield for windowpanes for houses, automobiles, etc., or that is, effective for reducing heat load due to sunlight. For reducing heat load due to sunlight, it is necessary to prevent transmission of sunlight rays falling within any of the visible range or the infrared range of the sunlight spectrum. In particular, windowpanes for automobiles are required to have high transmittance of visible light from the safety viewpoint, and are additionally required to have a high level of heat shieldability. In some countries, there is a move to control Tts (total solar energy transmitted through glazing).
Laminated glass coated with a special metallic film capable of blocking out thermal radiations, which is referred to as Low-E pair glass, is often used as eco-glass having high heat-insulating/heat-shielding ability. The special metallic film may be formed by lamination of plural layers, for example, according to a vacuum-deposition method. The special metallic film formed through vacuum deposition is extremely excellent in reflectivity, but the vacuum process is nonproductive and its production cost is high. In addition, when the metallic film is used, it also blocks electromagnetic waves; and therefore in use in mobile telephones and the like, the metallic film may causes radio disturbance; or when used in automobiles, there may occur a problem in that ETC (electronic toll collection) could not be used. Not only for evading the problem of radio disturbance but also from the safety viewpoint, windowpanes for automobiles are required to have high transmittance of visible light.
There has been proposed the process employing a cholesteric liquid crystal phase. For example, as disclosed in Japanese Patent No. 4109914, a λ/2 plate, of which both surfaces have a cholesteric liquid crystal layer, can selectively and effectively reflect a unidirectional circular polarized light of a wavelength within a range of from 700 to 1200 nm.
JP-T 2009-514022 discloses an IR-reflective article having a cholesteric liquid-crystal layer. Many trials of using a laminate of plural cholesteric layers in a liquid-crystal display device have been made, and concretely, there are known many trials of efficiently reflecting a light falling within a visible light range. For example, Japanese Patent No. 3500127 discloses examples of a lamination of a lot of cholesteric layers. In the examples described in Japanese Patent No. 3500127, the lamination was produced by repeating the step of applying a cholesteric liquid-crystal material and the step of curing thereof on the alignment layer formed on the glass substrate.
In Japanese patent No. 4008358, there has been proposed a method of irradiating a liquid crystal mixture, which is disposed between two substrates, with UV rays for curing thereof, to form a broadband cholesteric liquid-crystal film. However, in the patent, there has been no method for producing any multilayered film. Japanese Patent No. 3745221 discloses a circular-polarized light extraction element having plural of liquid crystal layers, which are formed by three-dimensional crosslinking liquid crystal molecules in cholesteric regularity and are stacked in the predetermined lamination-condition. In the examples described in Japanese Patent No. 3745221, the element having a multilayered structure was produced by applying the liquid crystal material to the glass substrate according to a spin-coating method, and then three-dimensional crosslinking the liquid crystal molecules to form each of the layers.
Japanese Patent No. 2743117 discloses a method for producing an optically active element comprising a step of transferring an optically active layer formed of liquid crystal polymer on an alignment substrate onto a transparent substrate. The method, disclosed in the patent document, may achieve the weight saving and the thinning of the optically active element by transferring the optically active layer formed on the alignment substrate onto the transparent substrate, and may enhance the industrial worth of the element. In the examples described in the patent document, the optically active layer formed on the polymer film was transferred onto another polymer film having the same thickness, or onto the polymer film onto the glass substrate.